Clonal cells of the mouse neuroblastoma C1300 that possess high concentrations of tubulin and whose content of microtubules can be directly regulated by growth conditions, will be used to determine the role of microtubule associated proteins in the initiation, elongation and stabilization of microtubules during nerve cell differentiation. The possibility that these accessory proteins regulate where and when tubulin polymerization occurs within the cell will also be examined. Additional investigations will be a continuation of current studies directed at the localization and characterization of the GTP binding sites in brain tubulin using photo-affinity labeling reagents.